I agree about the boot loader problem, but it can be re-written from the
live disk. His larger problem is protecting his data and reorganizing his
drive.
I would backup in any case. The best thing would be as LAC suggests. Install
Windows first. Windows likes to be number one and it ignores other OSes and
file systems. It will even want to format over your ext file system. For
that reason it is best to do your partitioning from Linux which identifies
other file systems, unless you know what you are doing and are confident.
An experienced Linux user could keep Ubuntu and install Windows last. It is
not that hard. However a newbie might prefer to backup and start fresh.
There are additional advantages perhaps not thought about when you first
installed such as setting up a separate home partition.